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Astron. Astrophys. 358, 451-461 (2000)

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4. Structure of the VLBI jet

4.1. Outer jet structure

A map of 0202+149 at 8 GHz, tapered by down-weighting the long-baseline data, shown in Fig. 9, reveals a core-dominated structure with a faint, quasi-linear jet extending out to [FORMULA] mas. Our high-dynamic range 22 GHz map (Fig. 10) shows that the more compact part of this diffuse jet is resolved into three components at a distance of 5.2[FORMULA]0.1 mas from the core and at position angle (PA) from [FORMULA] to [FORMULA]. The structure is similar to that of the 15 GHz image of Kellermann et al. (1998). Also, all 43 GHz maps (Table 3) show the presence of a stationary component at a distance of 5.15[FORMULA]0.05 mas at PA [FORMULA] (labeled as D in Fig. 10). Taking into account the double structure of the inner ([FORMULA] mas) region, the jet seems to be resolved in the tranverse direction (perhaps with a modest bend), rather than bent by 90o as suggested by Kellermann et al. (1998), although a sharp bend might be present in the inner 1 mas (see below).

[FIGURE] Fig. 9. A tapered map of 0202+149 at 8  GHz; the contours start at level 0.5% and increase by factors of 2; the map peak is 2.04 Jy/beam.

[FIGURE] Fig. 10. Hybrid map of 0202+149 at 22 GHz; the contours start at level 0.25% and increase by factors of 2; the map peak is 1.19 Jy/beam.

4.2. Inner jet structure

For the period from January 1995 to September 1996, more than 10 maps have been obtained from geodetic 8 GHz VLBI observations. The best 8 maps are presented in Fig 11. We obtained best agreement with observations with a three-component model: a core, labeled as A, and two-components (B and [FORMULA]) of a curved jet. The parameters of the gaussian model components are given in Table 2, where the first column is the epoch of observations, the second designates symbols used for the each component, the third gives the component's flux, the fourth is the distance of the component from the core (R), the fifth, the position angle of the component ([FORMULA]), and the sixth, the size of the component (a).

[FIGURE] Fig. 11. Hybrid maps of 0202+149 at 8 GHz convolved with a circular Gaussian beam of FWHM similar to the resolution in the direction of the jet as seen in Fig. 9; the contours start at level 0.5% and increase by factors of 2; the map peak is 0.94 Jy/beam in 1996.10.


Table 2. Model parameters at 8 GHz

The inner structure of the jet, more clearly seen in the high resolution 43 GHz map (Fig. 12), suggests either strong curvature or complex cross-sectional structure.

[FIGURE] Fig. 12. Hybrid map of 0202+149 at 43 GHz; the contours start at level 0.5% and increase by factors of 2; the map peak is 0.334 Jy/beam.

Hybrid maps at 43 GHz at 4 epochs of observation are shown in Fig. 13. The results of model fitting are summarized in Table 3. The brightest component is designated by the letter A and is identified as the core. The size of the core is in the range of 0.05-0.12 mas according to our model fits. The 43 GHz jet consists of at least two components, labeled as B and C. The most distant feature detected at 43 GHz is the aforementioned component D, which seems to be stationary.

[FIGURE] Fig. 13. The hybrid maps of 0202+149 at 43 GHz, convolved with a circular Gaussian beam of FWHM=0.15 mas, similar to the typical resolution along PA [FORMULA]; the contours start at level 0.5% and increase by factors of 2; the map peak is 0.42 Jy/beam in 1995.59

In Fig. 14 the positions relative to the core A of the inner jet components B (diamonds) and C (triangles) are plotted for 8 GHz (solid), 22 GHz (half-open) and 43 GHz (open). The data at the different frequencies are in reasonable agreement and, taken together, indicate that the trajectories of the components are rather stable.

[FIGURE] Fig. 14. Rectangular coordinates of components B (diamonds) and C (triangles) at 43 (open), 22 (half-open) and 8 (solid) GHz relative to the core.


Table 3. Model parameters at 43 GHz

Displacement between the components at 43 and 8 GHz may correspond to a frequency dependent shift in position of the core due to gradients in opacity (Gómez et al 1997; Lobanov 1998), as suggested also by time delays between the mm- and cm-wave light curves; however, the resolution at 8 GHz is insufficient for a definite conclusion. Fig. 14 shows also that there is a difference of up to [FORMULA] in the position angles of the inner jet components, which could be explained by strong curvature of the inner jet. The presence of stationary component D at a distance [FORMULA] 5.2 mas and PA [FORMULA] suggests that the jet wiggles through one more turn of a few tens of degrees toward its initial direction beyond 1 mas from the core. The 8 GHz (and 22 GHz) component B is resolved on the 43 GHz map into components B and C. The 8 GHz (and 22 GHz) component [FORMULA] is not seen at 43 GHz, probably due to a low surface brightness. Despite fluctuations in the position of component C, it is also rather stationary.

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© European Southern Observatory (ESO) 2000

Online publication: June 8, 2000